This invention relates to a video display apparatus suitable for displaying a video picture with an aspect ratio different from that of the video screen, and more particularly to a video display apparatus suitable for displaying standard TV pictures (with, say, 525 scanning lines and the aspect ratio of 4:3) on the screen of a display apparatus for high-quality TV system (called Hi-Vision TV or HDTV with, say, 1125 scanning lines and the aspect ratio of 16:9).
Ordinary TV receiver screens have the aspect ratio (the ratio of frame width to frame height) of 4:3 and the fluorescent surface is also sized to this ratio. On the other hand, for the high-quality TV such as HDTV or Hi-Vision TV (generally called HDTV (high definition TV) hereafter), new aspect ratios such as 5:3 and 16:9 are being proposed. To receive such high-quality pictures requires a wide-screen television receiver. In addition to displaying the HDTV pictures, the wide-screen television receiver is also required to receive ordinary TV images (aspect ratio of 4:3) because of economy and ease of handling. When, however, a picture with an aspect ratio of 4:3 is displayed on a fluorescent screen with an aspect ratio of 16:9, blank portions 2, 2', 2" where no picture is displayed are formed on one side or both sides of the screen as shown in FIGS. 1 to 3. What matters in this case is that if such a condition is allowed to persist for many hours, the fluorescent surface on the picture display portions 1, 1', 1" of the screen will deteriorate or its lumianance level will be reduced as a result of the browning phenomenon of the panel glass. This in turn produces variations in brightness on the screen when the entire fluorescent surface is used for displaying the HDTV picture for which the display is originally intended. The luminance variation conspicuously shows along the boundary line between the ends of the conventional 4:3-aspect-ratio picture area and the blank portions, thereby spoiling the view when the HDTV picture is shown on the entire screen. Thus, the quality of the HDTV picture as a whole is badly impaired.
The technique of receiving video signals of both the high-quality TV (HDTV) system and the standard TV system and of switching between the two systems are already known and thus not described here. Description of such a technique may be found in the Japanese Patent Application Laid-Open No. 26173/1988.
Apart from those proposed by this invention, the conventional techniques to reduce the level of nuisance the observer may feel from the blank portions, which occur when ordinary TV pictures are displayed on the wide-screen HDTV receivers, include the following.
That is, the Japanese Patent Application Laid-Open No. 33087/1986 describes a technique in which when ordinary TV pictures are displayed on the screen, frame lines are provided on the boundary between the picture display portion and the blank portions to trim away the unnecessary blank portions that occur on one side or both sides of the screen due to the difference in aspect ratio between the two types of pictures.
The Japanese Patent Application Laid-Open No. 128670/ 1987 shows a technique in which a mask plate is provided on the blank portions of the screen to cover them from view when ordinary TV pictures are displayed on the wide-screen HDTV receiver because the blank portions on the fluorescent screen left shown on each side of the picture display portion will spoil the view of the screen.
In the Japanese Patent Application Laid-Open No. 26173 /1988, a technique is introduced in which when ordinary size TV pictures are shown on the wide-screen HDTV receiver, an arbitrary mono-color is displayed on the blank portion on the screen to give an improved or pleasant appearance of the screen.
These conventional practices, as shown above, take measures only with respect to the impaired view of the overall screen when ordinary TV pictures are displayed on the wide-screen HDTV receivers.
However, these measures do not solve another important problem that occurs with TV display apparatuses that can display both the HDTV pictures and the ordinary TV pictures. This problem is the imbalance in luminance level on the fluorescent surface between the picture display portions and the blank portions on the screen after the ordinary TV pictures have been displayed on the wide-screen HDTV receivers for many hours. In other words, when the HDTV picture is displayed on the screen, which has the above luminance imbalance, a sharp difference in luminance shows so conspicuously as to spoil the quality of the entire picture on the screen. The conventional practices have not provided any solution to this important problem.